2 Answers
2

Commenting on any posts is a privilege that only users with a reputation of at least 50 (on Stack Overflow) have.
If users with a social network account (and without a Stack Exchange account) could comment on answers, then the minimum reputation required to comment on any post would be 1. I don't think it is a good thing; we don't need to increase the number of comments, especially from users who are not minimally involved in any Stack Exchange site.

If you are referring to users with an account on Stack Exchange and a social network, I don't see the reason why they should use that account instead of the one they already have on Stack Exchange.